You would have to use a client access list in postfix to get around this issue. Below is an example of what you would have to put in your postfix main.cf file, in the smtpd_recipient section. You just have to create a file called /etc/postfix/goodip and add the IP and OK to it eg. (2.2.2.2 OK) and then postmap it (postmap /etc/postfix/goodip). Then add the below string to postfix and do a postfix reload. This should now allow their IP through postfix. You can google check_client_access to get a better idea of the usage and syntax.